Para-xylene is an important starting material for manufacturing terephthalic acid, which is itself a valuable intermediate in the production of synthetic polyester fibers, films, and resins. These polyester materials have many practical, well known uses, such as in fabrics, carpets, and apparel.
One known route for the manufacture of para-xylene is by the methylation of benzene and/or toluene. For example, U.S. Pat. No. 6,504,072 discloses a process for the selective production of para-xylene which comprises reacting toluene with methanol under alkylation conditions in the presence of a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa) wherein said porous crystalline material has undergone prior treatment with steam at a temperature of at least 950° C. to adjust the Diffusion Parameter of said material to about 0.1-15 sec−1. The reaction can be carried out in a fixed, moving, or fluid catalyst bed.
In addition, U.S. Pat. No. 6,642,426 discloses a process for alkylating an aromatic hydrocarbon reactant, especially toluene, with an alkylating reagent comprising methanol to produce an alkylated aromatic product, comprising: introducing the aromatic hydrocarbon reactant into a reactor system at a first location, wherein the reactor system includes a fluidized bed reaction zone comprising a temperature of 500 to 700° C. and an operating bed density of about 300 to 600 kg/m3, for producing the alkylated aromatic product; introducing a plurality of streams of said alkylating reactant directly into said fluidized bed reaction zone at positions spaced apart in the direction of flow of the aromatic hydrocarbon reactant, at least one of said streams being introduced at a second location downstream from the first location; and recovering the alkylate aromatic product, produced by reaction of the aromatic reactant and the alkylating reagent, from the reactor system.
The reaction of toluene and methanol, particularly using the highly steamed catalyst described in the '072 patent is highly selective to the production of para-xylene. However, in addition to para-xylene the reaction product contains water, as a necessary by-product of the substitution of a hydrogen group of the benzene ring by the methyl group of the methanol. Other side reactions generate small quantities of oxygenated organic species, many of which are organic acids, such as formic acid, acetic acid and alkyl phenols (such as, methyl, dimethyl and ethyl phenols). Thus the effluent from a toluene methylation reaction includes product xylene, unreacted toluene, light gas products, heavier aromatic species and an aqueous vapor phase. Where the reaction is conducted in a fluidized bed, such as disclosed in the '426 patent, the effluent will also contain catalysts fines which are not recovered by the catalyst separation system of the reactor. These catalysts fines mostly end up in the aqueous phase which, by virtue of the presence of the organic acids, has a low pH. Such an environment is conducive to partial dissolution of the catalyst fines rendering them “sticky” and difficult to separate from the water/oil mixture. This solids separation problem is accentuated by the fact that the solids are in low concentration requiring the processing of large volumes of liquid in the solids recovery step (such as, filtration).
There is therefore a need for an effective process for recovering catalyst fines from the reaction effluent of a fluid bed methylation process that avoids the processing of a low pH aqueous phase having a very dilute catalyst solids concentration. According to the invention, this is provided by contacting the reaction effluent vapor with a liquid hydrocarbon quench stream so as to condense a controlled, minor part of the effluent vapor and produce a condensate which contains at least some of the catalyst fines and which is substantially free of an aqueous phase. After concentration of the solids in the condensate, the catalyst fines can be recycled back to the methylation process preferably using a flush stream containing methanol and/or the aromatic reagent of the process, namely benzene or toluene.